This invention relates to variable speed constant frequency electrical power systems and more particularly to such systems which include an auxiliary DC power output.
Reliability in aircraft electrical power generation systems is a critical item for all types of airplanes. Sophisticated airplanes utilize a "fly by wire" system wherein the airplane flying control surfaces are controlled by a computer. A total power outage would result in the loss of the computer and a subsequent loss of the airplane. Backup power systems for such an aircraft are mandatory. Therefore, it is apparent that the emergency power systems and below idle power systems are always required.
Two types of aircraft electrical power generation systems have found wide acceptance. These include systems of the constant speed generator/mechanical transmission type and newer electronic variable speed constant frequency (VSCF) systems. Both of these systems lack the ability to provide AC power when they are operated below their normal design speed range. Although lower speed capability can be added, it requires a large weight and size penalty. Therefore the demand for such capability from the primary AC power source has been small. Auxiliary power is generally furnished by battery systems which supply a low voltage of for example 28 volts DC.
One type of VSCF system includes a generator which operates at variable speeds to supply a DC voltage to an inverter by way of a pair of DC-link conductors. Previous approaches to providing auxiliary power from DC-link systems were heavy, costly and did not take advantage of the inherent capability of the system. These approaches utilized a separate isolation transformer, rectifier and filter. In some cases, the regulation was done with an active switching regulator. Each of these approaches resulted in the addition of significant weight to the system.